Add linux-arm64 builders

[dagood]

This starts off simple--GOARCH=arm64--but there are a few things to solve.

• Always build the _core tool for the host OS, not the target. We need to be able to execute the tool. This is not hard (just clear out the value to make Go detect it), prototype here: microsoft/main...dagood:dev/dagood/cross-support
• Set up cross-compile C++ toolchain to build Go with CGo enabled. Otherwise this comes up when trying to install/build the race runtime:

  ...
  Building packages and commands for host, linux/amd64.
  Building packages and commands for target, linux/arm64.
  ---
  Installed Go for linux/arm64 in /home/dagood/git/go
  Installed commands in /home/dagood/git/go/bin
  ---- Building race runtime...
  ---- Running command: [../bin/go install -race -a std]
  go install: -race requires cgo
  panic: exit status 2
  
  goroutine 1 [running]:
  main.main()
          /home/dagood/git/go/eng/_core/cmd/build/build.go:60 +0x2c9
  

• How will we run tests? (Cross-compile then test in a real machine? Where are these machines available--Helix?) (Probably want to split this to another issue.)

[dagood]

I ran some hacky builds with our pipeline on borrowed, real arm64 hardware and got some configs working on 1.17 (picked for branch stability) on an arm64 Mariner 1.0 image I built:

• https://dev.azure.com/dnceng/internal/_build/results?buildId=1629937&view=results - linux-arm64-buildandpack
• https://dev.azure.com/dnceng/internal/_build/results?buildId=1631486&view=results - linux-arm64-test
  • I had to pick a fix from upstream into 1.17 and apply a theoretical flakiness fix to get the tests to go all the way.

[dagood]

.NET Docker made these changes to run on a new arm64 pool that we will need to do in go-images (but ideally take it semi-automatically by updating the entire infra to latest): dotnet/docker-tools#983

[dagood]

Completed by microsoft/go-images#95